An image reading apparatus has long been employed in such devices as a fax machine, a copy machine, a scanner, a bill discriminating device, a testing device, and a fingerprint authentication device. Reading targets by the image reading apparatus cover a wide variety of items such as a manuscript, a book, a page of a magazine, a document, a picture, a photograph, a slide film, a film, a banknote, a bond, a board, an electronic component, and a fingerprint. The image reading apparatus obtains an image of a reading target in such a way that a photodetector (sensing element) receives reflected light from the reading target. Depending on the property of the reading target and the purpose of the reading, there may be a case where reflected light received by a photodetector is light having wavelengths not only of visible light, but also of non-visible light. Thus, in a light source installed inside or outside the image reading apparatus, that is, in the light source (illuminating device) that irradiates the reading target with light in order to receive reflected light from the reading target, a light source element that not only emits visual light but also emits light having wavelengths other than that of visual light may be used in some cases. As the light source element for the light source (illuminating device), a light source element (point light source) such as an LED (Light Emitting Diode) or organic EL (Electro Luminescence) is frequently used.
In general, a direction to which a reading target is fed is referred to as a sub-scanning direction (feed direction) of the image reading apparatus. A direction that intersects with (that is frequently orthogonal to) the sub-scanning direction is referred to as a main scanning direction of the image reading apparatus. Photodetectors in the image reading apparatus are disposed frequently along the main scanning direction. A reading depth direction (depthwise direction) of the image reading apparatus is a direction that intersect with (that is frequently orthogonal to) the main scanning direction and the sub-scanning direction. Further, regarding a light source used in the image reading apparatus, in a case where light from a light source element is guided in a light guide and then a reading target is irradiated with the light and also the light guide is extending rod-shaped, the relation between a longitudinal direction and a transversal direction in the light guide is defined as follows. The longitudinal direction in the light guide corresponds to the main scanning direction in the image reading apparatus and the transversal direction in the light guide corresponds to the sub-scanning direction in the image reading apparatus.
A configuration of the light source in which light emitted from a light source element is guided in a light guide and then a reading target is irradiated with the light is as follows. The light guide used in the image reading apparatus includes a light scattering area (light scattering portion) that extends along the longitudinal direction and a light emitting surface portion in a surface opposite to the light scattering area. In such a light guide described above, incident light from an end portion in the longitudinal direction propagates in the longitudinal direction and is scatteringly reflected in the light scattering area, and linearly radiated light is emitted from the light emitting surface portion opposite to the light scattering area so as to irradiate a reading area (for example, refer to Patent documents 1 to 6). Note that the reading area is an irradiated portion on a reading surface, irradiated with light from the light source.
In addition, as a light guide used for an image reading apparatus, there also exists a light guide in which, without the use of the light scattering area (light scattering portion), light emitted from a plurality of light source elements (point light sources) disposed in the longitudinal direction is transmitted, and linearly radiated light is emitted from the light emitting surface portion so as to irradiate the reading area (for example, refer to Patent documents 7)